Why are knobs called "pots" by some sound designers?

Pot is short for "potentiometer". It's the doodad behind the panel, connected to the knob, that divides the voltage ("potential") between two ends of an element. It does not mean knob, nor does it mean dial. Loosely also used to refer to a rheostat, which is an adjustable resistance rather than two resistances that are used to divide voltage.

"Pot" comes from "Potentiometer".

http://en.m.wikipedia.org/wiki/Potentiometer

These are under the knobs and are the active part.

A potentiometer always has three electrical terminals and so forms a true adjustable potential divider (potential ≡ voltage). This passes a fraction of the signal (i.e. from 0 to 100% depending on how far the attached knob is turned or slid) on to the next stage of amplification in the audio mixer, radio, TV set, drill speed control, lamp dimmer or almost any other electronic device with a "knob" on it :)

This is as opposed to the more general term "rheostat" or variable resistor which usually only has two terminals.